This invention relates to an air-fuel ratio control method for internal combustion engines, and more particularly to a method of this kind which is adapted to control the air-fuel ratio of the engine under a high load operating condition.
Conventionally, there has been employed an air-fuel ratio control method which controls the air-fuel ratio of a mixture supplied to an internal combustion engine to a stoichiometric ratio or a value close thereto by means of feedback control when the engine is under a low or middle load condition, and interrupt the feedback control and enriches the mixture when the engine shifts into a higher load condition to thereby prevent an excessive rise in the temperature of the engine by so-called cooling-by-fuel. However, this method has suffered from drawbacks, such as increased fuel consumption and degraded exhaust emission characteristics.
In order to eliminate such drawbacks, a method has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 59-128941, which makes the mixture leaner than a required rich air-fuel ratio for a predetermined time period after the engine operating condition has shifted to a predetermined high load operating condition, and enriches the mixture to the required rich air-fuel ratio after the lapse of the predetermined time period, and a method by Japanese Provisional Patent Publication (Kokai) No. 57-24435, which enriches the mixture after a predetermined high load operating condition of the engine has continued for a predetermined time period.
In general, if the so-called feedback control region, in which the air-fuel ratiO is controlled to a stoichiometric ratio or a value close thereto by means of feedback control, is expanded toward the higher load side in order to improve the exhaust emission characteristics, the amount of the mixture supplied to the engine increases when the engine is operating under a relatively high load condition within the feedback control region, so that the amount of heat generated by the engine increases to increase the temperature of exhaust gases. However, according to the above prior art method employing leaning of the mixture before the lapse of the predetermined time period, the mixture is enriched after the predetermined time period has elapsed from the time the engine operating condition shifted to the predetermined high load operating condition. Therefore, if the engine operating condition has shifted to the predetermined high load operating condition after the engine continued operating under the relatively high load condition within the feedback control region, the temperature of exhaust gases becomes very high before the predetermined time period elapses, which shortens the life of an exhaust gas-purifying device arranged in the exhaust pipe of the engine.
Further, according to the above-mentioned prior art method employing enriching of the mixture after the lapse of the predetermined time period, the mixture is not enriched to such an extent as to effect cooling-by-fuel if the engine intermittently operates under the high load condition over time periods each of which is shorter than the predetermined time period, as shown in (1 ) of (a) of FIG. 13 (in which the high load operating condition is defined as a condition that an engine operating condition parameter for determining the high load operating condition is above a critical value). Accordingly, the temperature of exhaust gases continues to rise as shown in (2) of (a) of FIG. 13. As a result, the temperature of exhaust gases can exceed the maximum allowable continuous temperature for exhaust gases, and even further rise, without falling below the maximum allowable continuous temperature within the maximum allowable time period during which the engine can withstand a temperature between the maximum allowable continuous temperature and a limit temperature, so that in the worst case it rises above the limit temperature. In particular, this causes an excessive rise in the temperature of a catalyst of the exhaust gas-purifying device.